In 2008, Stanford University in the U.S.A. proposed an Open Flow protocol which uses an architecture in which forwarding plane and control plane are separated. In the architecture of the Open Flow protocol, a control plane entity takes use of a controller based on the Open Flow protocol to implement forwarding logics, while a forwarding plane entity takes use of a gateway based on the Open Flow protocol to implement controlled forwarding according to a flow table issued from the controller. The behaviour of the gateway is standardised. For example, after receiving a message, the gateway analyses the message to obtain the header, i.e. L2/L3/L4 related field values, and takes these values as the keywords for searching the flow table, and after an item in the flow table is matched, transforms the message field based on the instruction set in the content of the item, forwards the message to a certain logical or physical port according to the instruction of the item when the transformation is terminated. A Software Defined Network (SDN) is a further evolution of the Open Flow protocol. The SDN may implement a variety of complex network applications in the manner of software programming on the control plane without any change of the forwarding plane devices. Moreover, since the control plane takes use of universal servers and universal systems, universal software programming tools or computer programming languages such as Python may be used for implementing script programming. Thus, the implementation of novel network protocols through the forwarding plane becomes extremely simple and the deployment cycle of new technologies is reduced significantly.
An Evolved Packet Core (EPC) network is a novel packet core network introduced in the phase of 3GPP R8. The EPC leads to further flattened networks, in which devices on the forwarding plane are further divided into a Serving Gateway (SGW) and a PDN Gateway (PGW). Since the R8, the EPC has a series of enhanced functions, including introduced technologies such as traffic offload. These enhanced functions requires a period of 1 to 2 years for formulating new protocol specifications and then they are implemented by equipment suppliers, tested and deployed by operators. Thus, the introduction cycle of new network characteristics becomes long and costly. The Open Flow protocol aims at overcoming these problems. The introduction of a new characteristic of network may be achieved by only modifying the logic of the control network element, i.e. controller, in the Open Flow protocol, the controller controlling devices on the forwarding plane to implement this new function, which leads to a fast introduction of new network functions. However, the Open Flow protocol is currently mainly applied to layer-2 switching networks. It cannot support directly the network elements in EPC on which service control are needed. The introduction of the Open Flow mechanism will affect functions and control modes of existing user plane network elements in the EPC.
FIG. 1 is a schematic diagram illustrating the system architecture of a SDN EPC. The Unified Gateway (UGW) in FIG. 1 merely has user plane functions. All the control plane functions are implemented by the controller. The user plane functions includes: establishment, modification, assurance of Quality of Service (QoS) of a user plane tunnel, etc. The UGW may be divided into SGW and PGW according to its function under the logic control of the controller. The SGW and PGW are essentially UGWs with same functions, which enable the EPC to implement more flexible networking, and the controller may introduce application functions flexibly through an Application Program Interface (API) to enhance the capability of the EPC.
As the basic requirement of operators to a network, the implementation of a charging function should also be considered in the case of an EPC network based on SDN. In the prior art, a Charging Trigger Function (CTF), a Charging Data Function (CDF) and a Charging Gateway Function (CGF) are integrated in a PGW, which is responsible for collection of charging information and credit control, and supports the interface to the charging system network element, so as to implement functions of offline/online charging and content charging. In view of implementation, compared with UGWs in a SDN network, charging function packet gateways in the prior art require logical control operations related to charging in addition to a forwarding function, leading to complex logics and high cost without universality, thus being no longer adapted for the new generation mobile communication network in which the control and forwarding are separated.